1. Statement of the Technical Field
The inventive arrangements relate to methods for linearizing RF power amplifiers, and more particularly to a method for providing an envelope elimination and restoration (EER) amplifier with enhanced linearity.
2. Description of the Related Art
The migration of broadcast and other communications industries to complex digital waveforms has necessitated a degree of amplifier linearity that is unprecedented. Concurrently, there is a continuing demand for amplifiers that operate more efficiently and offer reduced power consumption. In the case of large transmitter installations, greater efficiency is important for reducing waste heat and costs. In other applications, such as that involving portable transceiver equipment, efficiency is important for reducing size, weight, and battery consumption.
One type of RF power amplifier which offers improved efficiency is the envelope elimination and restoration (EER) amplifiers. EER amplifiers are well known in the art and can achieve very highly efficient conversion of DC energy to RF energy for complex waveforms having a varying envelope. They operate by separately processing the envelope and phase information contained in a modulated input signal. The phase information is communicated to a power amplifier where it is amplified as a constant envelope signal. This permits such phase information to be amplified using highly efficient non-linear amplifiers. The envelope information contained in the input signal is restored to the phase information after the signal has been amplified.
Although highly efficient, EER amplifiers using Class E topologies are known to have poor linearity. This poor linearity causes significant amounts of signal distortion. For example, such distortion often arises from pulse-width modulator circuits that are used to control the output envelope voltage, and from switching non-linearities which exist in the circuit used for amplifying the phase information. The nonlinearities cause spectral re-growth (out-of-band noise), which leads to adjacent channel interference. It also causes in-band distortion, which degrades the bit-error rate (BER) performance for digital modulation waveforms. In order to comply with FCC spectral masks, reduce BER, and achieve acceptable amplifier efficiency, linearization is necessary.
Distortion associated with RF power amplifiers is often characterized by means of an amplitude to amplitude (AM-to-AM) modulation curve and an amplitude-to-phase (AM-to-PM) modulation curve. The AM-to-AM modulation curve shows the RF power amplifier gain as a function of the input power. The AM-to-PM modulation curve shows the output phase variation of the RF power amplifier as a function of the input power. It will be appreciated that AM-to-AM distortion and AM-to-PM distortion can adversely affect the performance of an RF communication system. For example, such distortion can make it difficult to recover symbols at a receiving end of a communication link.
One well known method for improving the linearity of RF power amplifiers is known as feedforward linearization. With feedforward linearization, an RF splitter is typically used to separate a source signal into two separate signals. These two signals include a amplifier input signal and a reference signal. The amplifier input signal is provided to the amplifier as an input. A directional RF coupler is used to obtain a sample of the distorted output signal from the RF power amplifier. The reference signal and the sampled output from the directional coupler are communicated to separate inputs of a 180° hybrid RF signal combiner. The 180° hybrid RF hybrid combiner subtracts the reference signal from the distorted amplifier output. The resulting output from the combiner is an error signal. The error signal is subsequently amplified so as to scale the error signal to equal the power level of any distortion contained in the distorted output signal from the RF power amplifier. The error signal is then subtracted from the distorted output signal of the RF power amplifier to remove the distortion from the output signal.
Feedforward linearization is effective at improving amplifier linearity. However, it has not been particularly practical for certain amplifier applications. For example, the relatively large magnitude of the error signal needed to improve the linearity of highly non-linear amplifiers can require a relatively high power RF amplifier for scaling the error signal. The necessity for such a relatively high power RF amplifier for scaling the error signal can reduce the overall efficiency of the amplifier system. Thus, feedforward linearization has been limited with regard to its usefulness as applied to highly non-linear amplifiers, such as the EER type amplifier.
Another limitation of feedforward linearization concerns bandwidth. In feedforward linearization systems, it is important for the error signal to be a highly accurate representation of the actual distortion produced by the RF power amplifier. A distorted error signal will not properly remove non-linearities from the output of the amplifier. However, in the case where the signals to be amplified are wideband RF signals inaccuracy of the error signal can occur. For example, such inaccuracies can result from amplitude and phase variations which exist across the operating bandwidth of the RF components used to form and process the error signal. As noted above, such RF components can include RF signal splitters and 180° RF hybrid combiner circuits.